Self-diffusion of n-paraffins in NaX zeolite

Abstract

Self-diffusion coefficients for intracrystalline diffusion of n-C_nH_2n+2 molecules (n= 1–18) adsorbed in NaX zeolite have been measured by means of the n.m.r. pulsed field gradient technique as a function of temperature (–150 to +220°C) and of sorbate concentration. While the self-diffusion coefficients decrease over two orders of magnitude, the activation energy remains constant with increasing pore-filling factor. Only for methane and ethane are significant changes in the activation energies observed. For the light paraffins (n⩽ 7) the pre-exponential factors of Henry constants and self-diffusion coefficients can be estimated from the activation energy of self-diffusion. For n 7 the pre-exponential factors of the self-diffusion coefficients of the sorbate (as well as of the liquid) obey the relation D(n)≈D(n₀) exp[–0.3(n—n₀)].

The concentration dependence of the self-diffusion coefficients can be described quantitatively using a modification of the free volume theory. In contrast to its original application to pure liquids where the mean jump lengths are assumed to be constant, the peculiarity of zeolitic diffusion is taken into account by equating the jump lengths with the cube root of the molecular free volume.

Additional information about the microdynamics of diffusion is obtained by including the results of n.m.r. relaxation analysis. On the basis of these investigations, values for mean molecular jump lengths can be determined. They are in satisfactory agreement with the values estimated on the basis of the free volume theory.